The present invention relates to electric circuit protective devices and, more particularly, to cable termination devices for protecting electric cables and/or other electrical equipment which are connected to overhead electric lines from the effect of lightning strikes on the overhead lines or other voltage surges.
Medium and high-voltage cables are normally connected to overhead lines by pothead insulator terminations, and to transformers by comparable bushing or elbow connector insulators. In each case, an insulated conductor passes through the device and surge protection is conventionally provided by a surge arrester electrically connected in parallel with the insulator. The arrester consists essentially of a body of voltage-dependent resistors made of semi-conductor material (typically metallic oxide ceramics) which exhibits a very high resistance and draws negligible current at service voltage but exhibits very low resistance and draws large currents (without generating proportionally large amounts of heat) when exposed to high voltage in a surge.
Surges due to lightning strikes can have extremely rapid rates of current increase (rates as high as 10.sup.11 amperes per second have been observed) and, under such circumstances, the inductance of even a few decimeters of a connecting lead, and the consequent voltage drop across it, can be very significant.
One of the problems in protective devices which has been discussed is the effect of the surge arrester leads. As the surge assumes a steeper rate of rise upon restrikes, the voltage across the arrester leads becomes more important. Lightning surges will prematurely age cables. The rate of aging increases as the magnitude of the voltage surge increases. Therefore, it is very important to limit as much as possible the magnitude of these surges.
It is an object of the present invention to provide a novel electrical protective device providing surge arresting properties for high voltage connections to overhead line in which the length of connecting leads is substantially reduced.
It is also an object to provide such a device in which the surge arresting and cable termination functions are provided in a common housing with highly desirable insulating characteristics.
Another object is to provide such a device which may be assembled readily and which will exhibit relatively long life.